Monitoring growth of ultrathin films via ion-induced electron emission

Abstract

H + , He + , and N + ions with energy of 25 keV are scattered under a grazing angle of incidence from a clean and flat Cu͑001͒ surface during deposition of ultrathin Co films. Making use of the ion-induced emission of electrons allows us to monitor growth of thin films via simple measurements of target current or from energy spectra of emitted electrons. The method provides excellent signals and is also applicable in the regime of poor layer growth.The physics of ultrathin films plays an important role in fundamental research as well as technological applications. As a prominent recent example, we mention the "giant magneto resistance" ͑GMR͒ effect 1,2 present in ultrathin films showing antiferromagnetic couplings. This effect is the basis for the function of reading heads used in modern magnetic hard disk drives or in sensor technology. 3 In particular for fundamental research, the preparation of defined ultrathin films in the monolayer ͑ML͒ regime is crucial. An established method is epitaxial growth of thin films via "molecular-beam epitaxy" ͑MBE͒ ͑Ref. 4͒, where atoms from an evaporator source are deposited on monocrystalline substrates.In the characterization of films, monitoring of growth plays an essential role, since thickness and growth mode determine decisively the structure and the functioning of the system. A widely used technique to inspect growth of thin films in the monolayer regime is "reflection high-energy electron diffraction" ͑RHEED͒ ͑Ref. 5͒, where keV electrons are scattered from the surface of the film under a grazing angle of incidence. In a simple picture, the intensity of reflected electrons depends on the "smoothness" of the film surface. Then, e.g., for "layer-by-layer growth" the morphology changes periodically with coverage, and intensity oscillations for reflected electrons are observed. Aside from the powerful features of RHEED, diffraction phenomena make the general interpretation of data a nontrivial problem.As alternative probes for monitoring film growth, scattering experiments with other microscopic particles are feasible which show a dependence of the backscattered yield on the morphology of the film surface. An interesting alternative to RHEED is scattering of fast atoms or ions instead of fast electrons. 6-10 This technique is similar to RHEED, however, it bears the advantage that the projectile trajectories can be described classically in terms of pure kinematical concepts. 11 For growth of thin semiconductor and metal films, grazing scattering of keV ions has been proven to be a powerful tool to study details on growth mode, island densities, critical island size, etc. 6-12 As an example, we mention studies on growth of ultrathin Co films on Cu͑001͒, where a deviation from a monotonic Arrhenius type of dependence for the island density as a function of inverse growth temperature is observed. 13 This unusual behavior of film growth is attributed to intermixing of film and substrate atoms at elevated temperatures as predicted from density-functional theory ͑DFT͒ calculation...

A setup for grazing incidence fast atom diffraction

“…11b) shows that at 480 • C, the growth oscillation is shifted by half a monolayer revealing the well-documented phase transition from the more arsenic-rich c(4x4) reconstruction, changing to the (2x4)γ at the onset of growth 10 . Note that similar results were obtained with RHEED or with keV ions at grazing incidence 56,57 outlining the dominant role played by ad-atoms on the surface specular reflectivity. The specific interest of GIFAD lies in the non-destructive behavior of atoms at low effective normal energy E ⊥ and the absence of charging effect.…”